This invention pertains to a new binder composition suitable for use in bonding a particulate material such as sand or other refractory or foundry material.
It is known in the art to employ phenol-formaldehyde resole resins to bond granular refractory material, e.g., in a foundry molding composition. For example, Quist, et al. U.S. Pat. No. 4,426,467 discloses such a composition which is curable at room temperature using a lactone as curing agent.
While such compositions are extremely desirable inasmuch as they are curable at ambient temperature, such resins may have unacceptable thermal properties, i.e., they may experience veining and tearing at high temperatures experienced when subjected to molten metal in the foundry. Such resins are also especially deficient for use in preparing refractory articles, which, in addition to being able to withstand extremely high temperatures (e.g., for use in furnaces), must optimally contribute carbon when sintered.
To obviate problems of thermal insufficiency and insufficient provision of residual carbon, phenolic novolac resins may be tried. However, novolac resins are often undesirable inasmuch as they (1) require heat to cure, (2) evolve noxious formaldehyde fumes, (3) possess practically no "green strength," or room temperature cure, and (4) exhibit high solution viscosities due to the high molecular weight of the novolac.
It is also known to use a phenol-formaldehyde novolac resin to bond a carbonaceous aggregate, such as is disclosed in Chandramouli, et al., U.S. Pat. No. 4,775,455 issued Oct. 4, 1988, and Chandramouli, et al., U.S. Ser. No. 210,726 filed June 23, 1988 (both assigned to the assignee of the present invention). In a preferred teaching in the former, a blend of novolac and resole resins may be employed, but requires thermal curing, or high acid levels.
Harris U.S. Pat. No. 4,357,45, issued Nov. 2, 1982 teaches the use of the tetramethylol derivative of 4,4' iso-propylidenediphenol, water and ethylene carbonate or propylene carbonate to produce a resin solution useful for binding fillers. Harris, however, does not utilize an alkaline cure and, in fact, in the Examples uses acid catalysis to cure the resins.
Japanese Laid Open Pat. Application Ser. No. 62-282743 (Dainippon Ink) teaches a phenol-formaldehyde resin binder in which the phenol-formaldehyde resin's average molecular weight is 300 -1,500, preferably 500 -1,100. The Dainippon Ink application teaches that the molecular weight should be as high as possible, and utilizes polymeric resole resins, not methylolated monomeric phenolic compounds.
Japanese Laid Open Applications Ser. Nos. SHO 63-40636 and 60-180904 teach potassium alkali resole type bisphenolic resins in an aqueous solution, which when mixed with sand and hardened by an organic ester, provide high strength molds. The potassium alkali resole type bisphenolic resin used in SHO 63-40636 and SHO 60-180904 is obtained by reacting formaldehyde with a previously methylolated bisphenol compound. SHO 60-180904 and SHO 63-40636 do not teach the use of a low molecular weight methylolated phenolic compound to crosslink a novolac resin, and 60-180904 requires a relatively high molar ratio of alkali/bisphenol of 1.2:1 to 2.4:1.